1. Field of the Invention
The present disclosure generally relates to the fabrication of integrated circuits, and, more particularly, to reticles for forming test patterns for feature cross-sectioning and methods of using the reticles and wafers fabricated using the reticles.
2. Description of the Related Art
In modern integrated circuits, such as microprocessors, storage devices and the like, a very large number of circuit elements, especially transistors, are provided and operated on a restricted chip area. Immense progress has been made over recent decades with respect to increased performance and reduced feature sizes of circuit elements, such as transistors. However, the ongoing demand for enhanced functionality of electronic devices forces semiconductor manufacturers to steadily reduce the dimensions of the circuit elements and to increase the operating speed and reduce the power consumption of circuit elements. The continuing scaling of feature sizes, however, involves great efforts in redesigning process techniques and developing new process strategies and tools so as to comply with new design rules.
As the critical dimensions of the circuit elements decrease, the dimensions of metal lines, vias and contact elements used to interconnect and access the circuit elements also decrease. The feature sizes are reduced to a level that the photolithography processes used to form the features approach physical limits with respect to the wavelengths of the radiation used to form the features. During the patterning of contacts, a photoresist or mask layer is formed above a dielectric or other layer on a substrate. An optical system transfers a circuit design printed on a reticle to the photoresist layer through an optical radiation illumination and projection system. An important aspect of the patterning process is ensuring that the right amount of radiation (dose) is transferred through the reticle to the photoresist layer. Underexposing the features can result in the incomplete formation of the features in the photoresist masking layer for forming the contact openings. Material remains in the bottom corners of the contact openings, a defect commonly referred to as scumming. When a subsequent contact etch is performed, the pattern is not effectively transferred to the dielectric or other layers below.
To evaluate the effectiveness of the photolithography, etch or additional process steps, a reticle may be fabricated that includes an array of contact patterning features. The reticle is used to pattern contact patterns in a photoresist layer on one or more test wafers. After exposing and developing the photoresist layer and performing any post-exposure etch processes to define the contact patterns in the underlying layers, the test wafer is cross-sectioned. It can be evaluated using metrology equipment to identify the critical dimensions of the contact patterns, sidewall profile and the presence of scumming defects. This characterization is commonly used during a research and development stage to evaluate the printability of a new device fabrication process. The resist patterned wafer may also be cross-sectioned before contact etch.
The design to be implemented on a reticle that is intended for the previously mentioned contact characterization techniques requires definition of the feature of interest's X-critical dimension (X-CD), Y-critical dimension (Y-CD), X-pitch and Y-pitch. An array of contacts is then generated with these X-CD and Y-CD characteristics. The contact array for cleaving in the X-direction is generated by incrementally shifting the contacts a set amount, for example 1 nm, in the Y-direction and shifting the contacts by the X-pitch in the X-direction. The contact array for cleaving in the Y-direction is generated by incrementally shifting the contact a set amount, for example 1 nm, in the X-direction and by shifting the contact by the Y-pitch in the Y-direction. The center-to-center distance between the contacts is slightly larger than the corresponding non-shifted pattern reference pitch. The corner-to-corner shift or offset ensures cleaving at least one contact along its largest dimension (diameter).
FIGS. 1A-1D illustrate the use of a contact test pattern for defining a reticle for patterning test wafers to allow characterization of a photolithography process for forming contact openings on a wafer. FIG. 1A is a diagram illustrating a conventional reticle layout for forming an array of contact patterns on a test wafer. A pattern 100 of contact features 105 is defined for the photolithography process being evaluated. The pattern 100 (collection of contacts) defines a pitch or spacing in the x-direction, Pitch-X. The pattern 100 allows for measuring a critical dimension in the y-direction, CD-Y. As shown in FIG. 1B, the pattern 100 is formed on a reticle 110 and is repeated in the y-direction with an offset in the x-direction between instances of the pattern 100, to facilitate wafer cross-section measurements. The reticle 110 is used to print a test wafer 115 to form contact patterns 120 in a photoresist layer 125, as illustrated in FIG. 1C. The test wafer 115 is cleaved along a vertical cleavage line 130 to cross-section the test wafer 115 to allow analysis of the contact patterns 120 patterned above a substrate layer 140, as illustrated in FIG. 1D.
In FIG. 1D, the contact patterns 120 intersected by the vertical cleavage line 130 are illustrated. The shifting in the x-direction provides a higher probability that any given vertical cleavage line will intersect the center of one or more of the contact patterns 120, where the most useful process, profile, remaining material thickness and contact width data may be collected. The contact patterns 120 can be measured to determine a critical dimension in the y-direction, CD-Y, due to the orientation of the pattern, the shifting in the x-direction, and the y-direction of the cleavage line 130. Outer regions away from the center of the contact opening 135 of the contact patterns 120 may exhibit scumming (photoresist residue) defects or a shallow sidewall angle. If a dielectric layer 140 below the photoresist layer 125 were to be patterned, the contact openings would not be formed properly, i.e., too small or not opened.
An alternate cleave pattern array implements both an x-shift and a y-shift. This allows flexibility in cleaving contact arrays in either the x- or the y-direction.
The conventional test pattern illustrated in FIGS. 1A-1D has several limitations. First, it only allows the measurement of a CD in one direction. In an actual device pattern, the pitch is typically not regular or uni-directional, as the pattern density changes in different regions.
One potential drawback of the current art, in which a pattern shift for neighboring contacts is applied in the x- and y-directions, is that if the shifts are large enough, then the cleave pattern will deviate significantly from the original non-shifted pattern. For example, the cleave pattern for an orthogonal array of contacts will no longer be very orthogonal, and it could print significantly differently from the orthogonal case, due to optical, resist and etch proximity effects.
The present disclosure is directed to various methods of forming patterns that may avoid, or at least reduce, the effects of one or more of the problems identified above.